Petroglyph
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Petroglyph clear sky chart from 2005-11-06 to 2025-08-05.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	12% 7% 3 6% 4 4 4 4 6% 7% 43%	125623
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 44%	122658
Transparency	Day	40% 3 11% 25% 18% 3	129218
	Night	41% 3 11% 24% 18% 3	127140
Seeing	Day	18% 13% 28% 24% 13% 4	16627
	Night	18% 12% 30% 27% 12% 1	26406

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 11% 5% 13% 5% 4 3 4 5% 5% 32%	3617
	11th to 20th	Day	9% 10% 3 7% 4 4 4 4 5% 6% 44%	4016
	21st to end	Day	17% 10% 4 9% 3 3 3 3 5% 5% 38%	4488
February	1st to 10th	Day	15% 12% 3 7% 5% 5% 4 3 5% 6% 35%	1800
	11th to 20th	Day	14% 10% 3 8% 5% 4 4 4 5% 6% 37%	1928
	21st to end	Day	13% 10% 4 7% 4 3 4 4 5% 6% 39%	1650
March	1st to 10th	Day	15% 11% 3 5% 3 3 3 2 3 4 47%	4610
	11th to 20th	Day	10% 7% 2 4 3 3 4 3 5% 7% 53%	4980
	21st to end	Day	12% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	5728
April	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 6% 8% 44%	2400
	11th to 20th	Day	17% 8% 2 5% 5% 5% 4 3 4 6% 42%	2374
	21st to end	Day	18% 8% 2 6% 5% 4 4 3 6% 6% 37%	2400
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 8% 8% 30%	5443
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 4 6% 8% 35%	6241
	21st to end	Day	15% 7% 3 7% 5% 5% 4 4 5% 6% 40%	6935
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 3 6% 8% 44%	2800
	11th to 20th	Day	7% 4 2 5% 4 5% 4 5% 7% 9% 48%	2800
	21st to end	Day	4 3 2 5% 5% 5% 5% 5% 7% 10% 48%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 3 6% 9% 45%	6360
	11th to 20th	Day	6% 4 2 7% 5% 5% 6% 5% 8% 11% 42%	6360
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 10% 41%	6434
August	1st to 10th	Day	6% 4 1 4 3 4 4 4 8% 9% 52%	4135
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 45%	3729
	21st to end	Day	7% 4 2 5% 4 4 4 3 6% 9% 53%	4180
September	1st to 10th	Day	13% 5% 1 4 4 4 4 4 5% 7% 49%	2280
	11th to 20th	Day	11% 5% 2 6% 3 4 5% 4 5% 8% 48%	2266
	21st to end	Day	9% 5% 2 6% 4 4 4 4 5% 7% 51%	2068
October	1st to 10th	Day	9% 5% 1 6% 4 3 3 3 4 6% 57%	1900
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 6% 53%	1900
	21st to end	Day	17% 7% 3 5% 3 3 3 2 4 5% 47%	2090
November	1st to 10th	Day	11% 6% 2 5% 3 3 4 3 3 5% 54%	1776
	11th to 20th	Day	12% 8% 3 5% 4 3 4 3 5% 6% 48%	1800
	21st to end	Day	14% 11% 3 6% 4 3 4 2 4 5% 44%	1735
December	1st to 10th	Day	10% 10% 2 5% 4 4 3 3 4 6% 50%	2610
	11th to 20th	Day	12% 10% 5% 8% 4 4 4 3 4 6% 40%	3008
	21st to end	Day	21% 16% 6% 8% 5% 4 4 3 4 4 26%	3982
January	1st to 10th	Night	16% 10% 7% 12% 5% 4 4 3 4 4 32%	6348
	11th to 20th	Night	11% 9% 4 12% 5% 6% 5% 3 4 4 37%	5946
	21st to end	Night	17% 11% 4 8% 4 4 3 2 4 4 38%	6490
February	1st to 10th	Night	18% 12% 4 8% 5% 4 3 3 4 5% 35%	2600
	11th to 20th	Night	14% 9% 4 9% 4 4 4 3 4 5% 41%	2357
	21st to end	Night	15% 10% 3 7% 4 3 3 3 5% 4 42%	1980
March	1st to 10th	Night	17% 13% 3 6% 4 3 3 3 3 5% 41%	5286
	11th to 20th	Night	11% 9% 2 5% 3 4 4 3 5% 6% 49%	4980
	21st to end	Night	15% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	5199
April	1st to 10th	Night	11% 7% 2 5% 4 3 4 3 6% 7% 48%	2000
	11th to 20th	Night	14% 8% 2 5% 4 3 4 3 5% 6% 45%	1975
	21st to end	Night	19% 8% 3 6% 4 4 3 4 4 6% 40%	2000
May	1st to 10th	Night	18% 8% 3 7% 4 4 4 4 6% 8% 34%	4449
	11th to 20th	Night	13% 7% 3 6% 4 4 4 4 5% 7% 43%	3654
	21st to end	Night	15% 8% 3 7% 4 4 4 4 6% 8% 38%	3942
June	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 7% 8% 48%	1600
	11th to 20th	Night	6% 4 2 3 4 3 4 4 7% 10% 54%	1600
	21st to end	Night	3 3 2 4 4 3 5% 5% 7% 9% 56%	1600
July	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 50%	3617
	21st to end	Night	5% 5% 2 6% 4 5% 5% 5% 7% 11% 45%	4420
August	1st to 10th	Night	5% 3 2 4 4 3 4 3 6% 9% 58%	2893
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	3100
	21st to end	Night	6% 3 2 4 3 3 4 3 5% 9% 57%	3476
September	1st to 10th	Night	12% 5% 3 5% 3 3 4 3 4 6% 53%	1900
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 6% 7% 53%	1900
	21st to end	Night	10% 5% 1 5% 4 3 4 4 5% 6% 54%	2103
October	1st to 10th	Night	7% 3 1 4 3 3 2 2 5% 8% 62%	2280
	11th to 20th	Night	13% 5% 2 4 3 3 3 2 4 6% 54%	2280
	21st to end	Night	17% 8% 2 4 3 3 3 3 4 6% 48%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 2 4 5% 57%	2522
	11th to 20th	Night	13% 7% 2 5% 4 4 3 3 4 6% 49%	2600
	21st to end	Night	17% 10% 2 5% 3 3 2 2 3 5% 46%	2660
December	1st to 10th	Night	13% 9% 2 7% 4 3 3 3 4 6% 47%	4580
	11th to 20th	Night	14% 9% 4 6% 4 3 4 3 4 6% 42%	5199
	21st to end	Night	21% 16% 4 8% 5% 4 4 3 5% 5% 25%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	54% 2 10% 21% 11% 2	3618
	11th to 20th	Day	41% 2 7% 26% 19% 5%	4016
	21st to end	Day	49% 2 10% 16% 16% 8%	4488
February	1st to 10th	Day	50% 3 11% 23% 12% 1	1800
	11th to 20th	Day	48% 4 11% 18% 15% 4	1938
	21st to end	Day	46% 3 8% 23% 15% 5%	1650
March	1st to 10th	Day	43% 3 5% 21% 20% 9%	4610
	11th to 20th	Day	32% 2 8% 26% 24% 7%	4980
	21st to end	Day	45% 3 14% 27% 10% 1	5730
April	1st to 10th	Day	38% 4 11% 26% 19% 1	2400
	11th to 20th	Day	45% 3 10% 22% 16% 4	2374
	21st to end	Day	48% 3 11% 24% 13% 1	2400
May	1st to 10th	Day	49% 5% 12% 22% 12% 1	5478
	11th to 20th	Day	47% 3 10% 24% 13% 1	6245
	21st to end	Day	44% 4 9% 22% 20% 2	6949
June	1st to 10th	Day	37% 5% 12% 28% 16% 2	2800
	11th to 20th	Day	31% 5% 12% 27% 22% 3	2800
	21st to end	Day	29% 6% 18% 26% 16% 5%	2800
July	1st to 10th	Day	36% 4 9% 29% 18% 2	6291
	11th to 20th	Day	35% 5% 15% 31% 12% 1	6285
	21st to end	Day	37% 5% 17% 30% 11% 1	6251
August	1st to 10th	Day	28% 4 16% 34% 18%	4121
	11th to 20th	Day	38% 5% 12% 26% 17% 2	3765
	21st to end	Day	30% 4 13% 36% 14% 3	4179
September	1st to 10th	Day	33% 4 11% 27% 19% 5%	2280
	11th to 20th	Day	35% 5% 7% 27% 24% 2	2266
	21st to end	Day	32% 5% 7% 28% 24% 3	2068
October	1st to 10th	Day	18% 2 6% 20% 45% 9%	3160
	11th to 20th	Day	23% 1 7% 21% 37% 10%	3140
	21st to end	Day	32% 2 6% 29% 27% 4	3428
November	1st to 10th	Day	34% 3 11% 26% 21% 6%	1776
	11th to 20th	Day	39% 3 8% 23% 21% 6%	1800
	21st to end	Day	44% 3 9% 25% 17% 1	1735
December	1st to 10th	Day	38% 2 9% 34% 16% 1	2610
	11th to 20th	Day	47% 2 9% 23% 17% 1	3005
	21st to end	Day	64% 2 12% 14% 8% 2	3982
January	1st to 10th	Night	57% 2 9% 21% 11% 1	6348
	11th to 20th	Night	51% 2 6% 24% 14% 2	5946
	21st to end	Night	51% 2 7% 20% 17% 3	6490
February	1st to 10th	Night	53% 3 9% 21% 13% 2	2600
	11th to 20th	Night	47% 3 10% 21% 17% 3	2358
	21st to end	Night	46% 3 10% 22% 16% 4	1980
March	1st to 10th	Night	48% 2 5% 21% 19% 4	5286
	11th to 20th	Night	38% 2 9% 23% 20% 8%	4980
	21st to end	Night	49% 3 13% 24% 10% 1	5199
April	1st to 10th	Night	36% 4 11% 27% 19% 3	2000
	11th to 20th	Night	40% 3 10% 25% 17% 5%	1975
	21st to end	Night	46% 4 9% 24% 16% 2	2000
May	1st to 10th	Night	49% 5% 14% 18% 12% 1	4479
	11th to 20th	Night	42% 4 14% 24% 15% 2	3655
	21st to end	Night	44% 6% 11% 24% 13% 2	3950
June	1st to 10th	Night	33% 4 16% 26% 18% 3	1600
	11th to 20th	Night	25% 7% 11% 30% 24% 4	1600
	21st to end	Night	23% 4 17% 30% 19% 7%	1600
July	1st to 10th	Night	34% 4 11% 28% 21% 2	3580
	11th to 20th	Night	31% 6% 14% 33% 14% 2	3572
	21st to end	Night	33% 4 20% 28% 13% 1	4292
August	1st to 10th	Night	23% 5% 18% 33% 20% 1	2883
	11th to 20th	Night	35% 5% 15% 25% 17% 3	3130
	21st to end	Night	25% 4 14% 36% 18% 3	3476
September	1st to 10th	Night	32% 4 9% 28% 21% 6%	1900
	11th to 20th	Night	30% 5% 10% 27% 26% 3	1900
	21st to end	Night	31% 5% 9% 24% 28% 4	2103
October	1st to 10th	Night	14% 2 7% 17% 50% 11%	3800
	11th to 20th	Night	23% 2 6% 21% 35% 13%	3779
	21st to end	Night	31% 2 7% 27% 27% 6%	4120
November	1st to 10th	Night	31% 3 11% 27% 23% 5%	2522
	11th to 20th	Night	37% 2 8% 24% 24% 5%	2600
	21st to end	Night	43% 2 10% 24% 19% 2	2660
December	1st to 10th	Night	41% 3 9% 31% 16% 1	4580
	11th to 20th	Night	45% 2 11% 22% 18% 1	5201
	21st to end	Night	62% 2 12% 15% 6% 1	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	26% 23% 36% 13% 2 1	300
	11th to 20th	Day	29% 27% 35% 8% 1	300
	21st to end	Day	20% 32% 36% 11% 2	330
February	1st to 10th	Day	26% 25% 36% 11% 1	300
	11th to 20th	Day	31% 28% 30% 9% 2 1	300
	21st to end	Day	32% 24% 35% 9% 1	249
March	1st to 10th	Day	27% 21% 40% 8% 3	300
	11th to 20th	Day	35% 21% 33% 9% 3	300
	21st to end	Day	32% 22% 33% 11% 2	441
April	1st to 10th	Day	22% 21% 35% 16% 6%	500
	11th to 20th	Day	28% 20% 29% 17% 5% 1	492
	21st to end	Day	23% 16% 37% 17% 6% 1	500
May	1st to 10th	Day	21% 19% 37% 16% 6% 1	503
	11th to 20th	Day	20% 13% 30% 24% 9% 2	686
	21st to end	Day	14% 11% 34% 25% 13% 4	789
June	1st to 10th	Day	13% 7% 27% 27% 21% 5%	730
	11th to 20th	Day	8% 5% 26% 39% 17% 4	730
	21st to end	Day	6% 7% 31% 37% 15% 4	730
July	1st to 10th	Day	12% 3 19% 28% 27% 10%	721
	11th to 20th	Day	6% 4 21% 40% 24% 5%	718
	21st to end	Day	9% 3 20% 36% 26% 6%	794
August	1st to 10th	Day	10% 3 19% 36% 24% 8%	688
	11th to 20th	Day	13% 3 22% 30% 26% 6%	479
	21st to end	Day	12% 2 15% 31% 29% 11%	539
September	1st to 10th	Day	14% 4 19% 30% 27% 6%	490
	11th to 20th	Day	10% 3 18% 33% 24% 12%	490
	21st to end	Day	13% 9% 22% 30% 18% 7%	468
October	1st to 10th	Day	13% 12% 27% 32% 12% 4	300
	11th to 20th	Day	20% 17% 33% 24% 5% 1	300
	21st to end	Day	30% 19% 30% 18% 2	330
November	1st to 10th	Day	18% 14% 40% 19% 7% 2	300
	11th to 20th	Day	25% 21% 29% 17% 7% 1	300
	21st to end	Day	26% 20% 33% 17% 4 1	300
December	1st to 10th	Day	19% 26% 33% 13% 7% 2	300
	11th to 20th	Day	27% 21% 33% 15% 3 1	300
	21st to end	Day	30% 24% 32% 11% 2	330
January	1st to 10th	Night	22% 22% 35% 18% 3	1000
	11th to 20th	Night	24% 25% 32% 16% 3	1000
	21st to end	Night	20% 22% 38% 18% 1	1100
February	1st to 10th	Night	29% 18% 35% 17% 2	1000
	11th to 20th	Night	23% 22% 37% 16% 2	802
	21st to end	Night	25% 21% 34% 19% 1	660
March	1st to 10th	Night	25% 18% 36% 18% 2	765
	11th to 20th	Night	25% 15% 36% 21% 3	600
	21st to end	Night	28% 15% 33% 20% 4	660
April	1st to 10th	Night	17% 19% 36% 25% 4	600
	11th to 20th	Night	20% 15% 34% 23% 8%	593
	21st to end	Night	25% 10% 38% 21% 6%	600
May	1st to 10th	Night	18% 8% 37% 28% 9%	600
	11th to 20th	Night	18% 8% 28% 31% 14% 1	591
	21st to end	Night	12% 6% 28% 33% 17% 3	650
June	1st to 10th	Night	11% 3 22% 40% 22% 1	600
	11th to 20th	Night	7% 2 24% 46% 20% 1	600
	21st to end	Night	4 2 27% 44% 22% 2	600
July	1st to 10th	Night	11% 1 13% 36% 33% 6%	585
	11th to 20th	Night	6% 3 15% 45% 28% 3	582
	21st to end	Night	8% 2 16% 41% 29% 4	657
August	1st to 10th	Night	7% 2 15% 40% 33% 3	579
	11th to 20th	Night	7% 2 16% 39% 33% 3	564
	21st to end	Night	7% 3 14% 35% 35% 6%	627
September	1st to 10th	Night	14% 2 13% 36% 29% 5%	570
	11th to 20th	Night	11% 2 13% 37% 30% 6%	570
	21st to end	Night	12% 7% 23% 30% 25% 4	755
October	1st to 10th	Night	9% 5% 28% 33% 23% 4	760
	11th to 20th	Night	18% 13% 30% 27% 11% 1	760
	21st to end	Night	25% 14% 32% 23% 6%	836
November	1st to 10th	Night	15% 11% 40% 25% 9% 1	780
	11th to 20th	Night	21% 16% 32% 25% 7%	800
	21st to end	Night	25% 16% 34% 20% 5%	860
December	1st to 10th	Night	25% 15% 34% 21% 5%	1000
	11th to 20th	Night	25% 14% 35% 22% 4	1000
	21st to end	Night	27% 19% 35% 17% 2	1100

Page generated at 2025-08-02 00:32:26. Version ac07

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